1. Field of the Invention
The present invention relates to amplifier circuitry comprising a class D amplifier for use in consumer and industrial electronic equipment.
2. Description of the Related Art
A known example of class D amplifiers for use in electronic equipment is a class D amplifier that is switched and driven with a constant current conduction ratio and at the same frequency as a switching power supply is switched and driven. This class D amplifier varies a phase difference of its driving voltages depending on an input signal and thereby produces a pulse-width controlled output (see Japanese Patent Laid-Open No. 2001-298330). With such a configuration, the class D amplifier achieves a reduced size and reduces beat noise due to the difference in driving frequency between the switching power supply and the class D amplifier. To cope with such beat noise, it is possible to control a switching frequency of the class D amplifier or a switching frequency of the switching power supply for each board so as to prevent the switching frequencies of the switching power supply and the class D amplifier or beat frequencies due to harmonics from falling within the audio-frequency band.
FIG. 4 is a block diagram illustrating a configuration of a conventional class D amplification device. A half-bridge power supply circuit 2 receives power supply from an AC power supply 5 and performs a switching operation at a duty ratio of 50% based on a clock received from an oscillation circuit 27 so as to supply a pulse voltage to an amplifier circuit 3. The amplifier circuit 3 performs a switching operation based on the clock received from the oscillation circuit 27. A phase difference modulator 28 generates pulses by changing a phase difference between the received clock and the pulse voltage received from the power supply circuit 2 depending on a sound input signal 6. The generated pulses from the phase difference modulator 28 are applied to a driving circuit 18 which performs switching in response to the pulses. As a result of this, the amplifier circuit 3 amplifies the power of the sound input signal 6 and outputs the amplified signal to a speaker 4. Since the switching operations of the switching power supply and the class D amplifier are performed at the same frequency, the class D amplification device in FIG. 4 can prevent beat noise from being caused by a difference in frequency between the switching power supply and the class D amplifier and by the harmonics of such frequencies.
With such a conventional configuration, however, there is a limit in reducing power loss because the switching power supply and the class D amplifier need to be driven at an equal driving frequency and cannot be driven at their optimum driving frequencies.